The cause of rheumatoid arthritis is unknown, and the number and importance of genetic factors outside the MHC are obscure. The North American Rheumatoid Arthritis Consortium (NARAC) was formed to comprehensively address this question using a variety of strategies including affected sib pair analysis and transmission disequilibrium testing. The consortium consists of ten collaborating centers which will carry out these specific aims: Specific Aim 1: They will mount a coordinated national effort to identify and collect 800 affected sib pairs with rheumatoid arthritis. Well-defined sibling pairs with RA will be collected by the consortium at eight cooperating centers. Entry into the study will depend on standardized criteria, documented by face to face clinical evaluation of every sib pair, hand x-rays read by a single radiologist, and centralized serological testing. Where possible, parents and unaffected siblings will be collected. A centralized clinical database, DNA, serum, and cell bank will be established for all affected sibling pairs and family members. Specific Aim 2: They will utilize allele sharing methods for genome wide screening to establish whether genetic regions outside the MHC are linked to susceptibility to RA. Screening for candidate genetic regions will be done by an analysis of allele sharing using a panel of highly polymorphic microsatellite markers, spaced at 10-15 cM intervals across the entire human genome. The large sample size should allow for the analysis of specific subgroups of sib pairs with phenotypes indicative of high genetic risk, namely early onset disease and male sex. Specific Aim 3: They will further investigate the candidate regions identified in specific aim 2 by association based methods, including transmission disequilibrium testing with closely spaced markers in the genetic regions of interest. In the course of specific aim 2, they expect to identify 5 to 10 candidate regions of interest. By using closely spaced polymorphic markers in these regions, they further expect to be able to define haplotypes 2 which can be tested for their association with disease susceptibility for RA by transmission disequilibrium testing.